1) Field of the Invention
This invention relates to a spark plug with a built-in pressure sensor, which is suitable for use in an internal combustion engine. In particular, this invention is concerned with a spark plug with a built-in pressure sensor which, when fitted in an internal combustion engine, is communicated with a combustion chamber of an associated cylinder of the internal combustion engine via a pressure-introducing channel formed in a metal shell of the spark plug.
2) Description of the Related Art
Coupled with the recent move toward internal combustion engines of higher performance, a need has arisen to systematically control combustion of an air-fuel mixture in each combustion chamber of an internal combustion engine. To permit detection of pressure variations due to combustion in each combustion chamber of the internal combustion engine so that knocking in the combustion chamber of the internal combustion engine can be detected, the specific fuel consumption can be improved and exhaust gas can be normalized, certain spark plugs with a built-in pressure sensor have been proposed, including one having a pressure sensor arranged in a mounting base of a metal shell of a spark plug and a gasket disposed on a lower face of the mounting base so that each variation in a tightening load of a spark plug due to combustion gas produced as a result of combustion of an air-fuel mixture in an associated combustion chamber is converted into an electrical signal to permit systematic control of the combustion of the air-fuel mixture in the combustion chamber Japanese Utility Model Publication (Kokoku) No. HEI 1-29554! as well as one provided with a pressure-introducing bore formed in a metal shell so that the pressure of combustion gas produced as a result of combustion of an air-fuel mixture can be guided directly to a pressure sensor disposed in a mounting base of the metal shell Japanese Utility Model Laid-Open (Kokai) No. SHO 62-96287!.
The conventional constructions referred to above are however not fully satisfactory. In the construction disclosed in Japanese Utility Model Publication (Kokoku) No. HEI 1-29554, what are detected by the pressure sensor arranged in the mounting base of the metal shell are variations in a tightening load of the spark plug on the mounting base of the internal combustion engine. Each variation in the tightening load of the spark plug is caused by a combination of a pressure--which is applied to an igniting part of the spark plug in a direction opposite to the direction of the tightening load by combustion gas produced as a result of combustion of an air-fuel mixture in the combustion chamber--and a pressure applied to the mounting base of the metal shell in a direction opposite to the direction of the tightening load of the spark plug by the combustion gas flowed through a space formed in a threaded portion of the metal shell. The space through which the combustion gas is allowed to flow in and out is extremely small so that, upon passage of the in-coming and out-going combustion gases through the space, the out-going combustion gas develops resistance to the in-coming combustion gas. This resistance therefore reduces the flow rate of the in-coming combustion gas, resulting in the formation of a deformation in the composite waveform. This leads to a lag in the control of the ignition timing or the like by the electrical signal outputted from the pressure sensor in response to the variation in the tightening load. When an output of the pressure sensor is actually measured by a pressure indicator on a commercial internal combustion engine, a shift is observed between an output waveform of the pressure sensor and a combustion pressure waveform measured by the pressure indicator. This construction is therefore accompanied by the drawback that it is insufficient in accuracy see FIGS. 6(a) and 6(b)!.
To overcome the above drawback, those having a pressure-introducing channel in a metal shell have been proposed as in Japanese Utility Model Laid-Open (Kokai) No. SHO 62-96287 so that the pressure of combustion gas produced as a result of combustion of an air-fuel mixture can be directly guided to a pressure sensor disposed in a mounting base of the metal shell. The formation of the pressure-introducing channel in the metal shell however requires difficult machining. Further, the pressure sensor is exposed directly to combustion gas of high pressure so that its mounting strength and its own durability must be high. In addition, it is impossible to fully eliminate the above-described lag in control which occurs due to resistance to an in-coming flow of combustion gas by an out-going flow of combustion gas through the pressure-introducing channel.
In the case of the construction that a pressure sensor itself is built in a mounting base of a metal shell of a spark plug, the whole spark plug must be replaced by a new one even if the pressure sensor alone fails. This leads to the disadvantage that an extra expense is unavoidable.